CHIP28, Channel-forming Integral membrane Protein of molecular weight 28kD, is the principal human water channel. CHIP28 is a very hydrophobic protein, and is prevalent in kidney, erythrocytes, colon and a large number of other tissues. It functions as a diffusion-limited passive pore that dissipates osmotic gradients across cell membranes. By sequence homology, CHIP28 has been shown to be a member of a family of channel/transport proteins. We seek to understand a molecular basis for water transport by solving the crystal structure of CHIP28.